Upon primary exposure to antigen, nave T cells bearing complementary antigen receptors (TCRs) undergo rapid activation and clonal expansion, leading to the generation of effector T cells. The vast majority (90-95%) of antigen-specific effector T cells that participate in the primary immune response undergo apoptosis (cell death) after antigen clearance. A small subset, however, survives and gives rise to long-lived memory T cells. Upon antigen re-encounter, memory T cells respond swiftly and robustly to eliminate the pathogen. Based on their tissue distribution, cell surface markers, and effector functions, memory T cells have been divided into two major subsets. Memory T cells expressing receptors such as CD62L and CCR7, which allow efficient homing to lymph nodes (LN), are termed central memory (TCM) cells; memory T cells lacking LN-homing receptors and preferentially residing in non-lymphoid tissues are termed effector memory (TEM) cells. Notably, both memory subsets display high levels of the marker CD44. Once established CD4 and CD8 memory T cell populations can be maintained for many years in vivo. Although early studies suggested that continued antigenic stimulation is required for maintaining T cell memory, neither cognate antigen nor MHC-encoded molecules appear to be required for the long-term survival of CD4 or CD8 memory T cells. Members of the tumor necrosis factor receptor (TNFR) family are involved in T cell costimulation and play a role in T cell survival. Mice deficient in OX40, CD27, or 4-1BB show greater defects in secondary compared to primary responses when infected with pathogens such as lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus, and influenza. Working primarily in mice, have developed a method of separating naive from memory T cells based upon their density. Using this new technique, we have made the following findings (especially with regard to CD8 T cell memory, which is important in anti-viral and anti-neoplastic responses): 1. Memory T cells exist in the G1 phase (high RNA, haploid DNA) of the cell cycle, whereas naive T cells exist in G0 (low RNA, haploid DNA). Stimulation of the former results in rapid release of high amounts of cytokines such as interferon alpha (IFNalpha). 2. Culture of purified memory T cells in the absence of other cells (resting) allows them to revert to G0. When they do, they respond to stimulation to the same degree and with the same kinetics as naive T cells. 3. Culture of memory T cells with purified dendritic cells (DC) prevents their reversion to G0 and maintains their characteristic potent effector responses. 4. Blockade of two TNFR molecules, CD70 and 4-1BB reverses the "protection" provided by DC. Furthermore, direct stimulation of the CD70 receptor on T cells, CD27, also maintains T cells in the memory state in the absence of any other cells. 5. All of these results were verified with memory T cells generated against lymphocytic choriomeningitis virus (LCMV) and identified by MHC-peptide "tetramers", which can identify individual antigen-specific cells. In preliminary studies, we found that human memory T cells also exist in G1, and in some cases we have been successful in "reverting" them to G0. Manipulating the memory T cell state of readiness might be of benefit in disorders involving chronic ongoing immune responses.